Olefin oxide gas sterilization of products made of such materials as rubber and plastics including polyvinyl chloride, polypropylene and polyethylene, using either pure ethylene oxide or propylene oxide, or an admixture with an inert diluent such as a fluorocarbon or carbon dioxide, is typically utilized for treating products which cannot withstand heat sterilization. Sterilization aids in the elimination of viable microorganisms or their endotoxin byproducts and decreases their associated infectious disease syndromes. After the sterilization period is completed, gas in the chamber is discharged to the atmosphere through an air pollution control device and the sterilized product removed. Due to the highly toxic nature of ethylene oxide and its classification as a suspected carcinogen by the National Institute for Occupational Safety and Health (NIOSH), special precautions must be taken to insure safe exposure levels to human receptors.
The Occupational Safety and Health Administration (OSHA) has a standard for worker exposure to ethylene oxide of 50 ppm as a time-weighted average (TWA) concentration for an eight-hour work shift. However, recently, due to the availability of additional data on potential carcinogenicity, OSHA proposed a new, more stringent standard of 1 ppm. It is expected that this proposed standard will be promulgated in 1984.
The major source of olefin oxide worker exposure in the plant is product degassing after removal of the product from the sterilizer. Different types of product absorb or entrap olefin oxide at different levels depending upon the nature of the product, e.g., physical dimensions and exposed surface areas of materials, the specific materials of construction, how the product is packaged and the specific sterilization cycle to which the product was exposed. Unfortunately, not all the absorbed or entrapped olefin oxide is removed in the sterilizer post-evacuation cycle. Hence, the remaining olefin oxide will degass after the product is removed from the sterilizer.
This degassing process can be relatively slow at typical plant ambient conditions and usually necessitates that the product be quarantined for a period of time. The requirement for product quarantining causes additional olefin oxide exposure to workers involved in transferring sterilized product into and out of the quarantine areas. The extent of product holding time in quarantine can also have a significant impact on product inventory requirements at the plant.
In order to reduce worker exposure to olefin oxide as a result of sterilized product degassing, it is first necessary to optimize the post-evacuation cycle. This will enable most of the olefin oxide to be removed from the product while it is still in the sterilization chamber. Time availability, however, usually places limitations on post-evacuation cycle length. Hence, product, when finally removed from the sterilizer, can still pose a worker exposure problem because of degassing. The answer to this problem is continuous aeration with hot air for extended periods of time in separate degassing rooms.
The use of heat to accelerate the olefin oxide degassing process is known. Unfortunately, the use of hot degassing rooms still does not eliminate the worker exposure problems since pallets must be constantly transported into and out of the room. Moreover, heating and ventilation requirements for these hot degassing rooms are relatively high and can be quite expensive, even with energy recovery included. The use of a hot degassing room also means that all sterilized product at the plant will be degassed under the same conditions. Conventional room design precludes optimization of the degassing process for different products. For example, shelf life problems may limit the maximum temperature to which a specific product can be exposed in the hot degassing room. Hence, this same temperature limitation would automatically apply to all other products in the room.
The use of hot degassing tunnels built directly adjacent to the sterilizer have also been used. In this arrangement, pallets are semi-automatically removed from the back of a double-doored sterilizer and transported into the tunnel. However, workers must still enter the tunnel to remove product and heating and ventilation requirements can still be quite high, although not as high as in a hot degassing room. Moreover, the tunnel must be built directly behind the sterilizer, the sterilizer must be double-doored and have a means to automatically or semiautomatically remove the pallets from the sterilizer and transport them into the tunnel. Furthermore, the maximum allowable degassing time is limited to the length of the sterilization cycle which in many cases does not provide sufficient time for acceptable degassing. Such limitations have prevented their widespread use.